1. Field of the Invention
This invention relates to the field of light detectors. In particular it relates to phototransistor light detectors with short turn off time and high sensitivity. A phototransistor light detector with the short turn off time and high sensitivity permits construction of light detectors capable of measuring rapid events.
2. Description of Prior Art
Light detection devices for measuring rapid events such as muzzle velocity projectiles are limited by their turn off time and sensitivity. There is usually an inverse trade off between the turn off time and sensitivity of phototransistors, even silicon phototransistors. In particular it has been difficult to provide shifts in voltage levels for phototransistors down to transistor-transistor logic (TTL). It is highly desirable to use phototransistors shifted down to TTL level because this is a common or standard logic level for most computers.
Previous methods to reach the TTL level have been resistor and capacitor networks. These methods are unreliable, cumbersome and time consuming as they require precise adjustment of variable resistors. An additional problem has been the high amount of false information which has resulted from attempting to take signals off near the initial part of the transition wave form and to reliably trigger pulses at levels close to the collector supply potential. Phototransistors alone do not give uniform response.
In summary, previous uses of phototransistors have been severally limited at low levels of light and the manufacturing tolerances which have given a factor of 2 in the variation of collector on-voltage. Thus there is a need for repeatable phototransistor turn-off points which are still sensitive to rapid events. The reason sensitivity and speed have a trade off is that increased sensitivity requires a larger active area which increases the capacitance and gives the circuit a longer time constant.